1. Field of the Invention
The invention relates to a rolling mill, especially a hot flat-rolling mill, having a pair of working rolls supported on a corresponding pair of supporting rolls either directly or via intermediate rolls and mutually axially displaceable to compensate for unevenness over the width of the material being rolled.
2. Description of the Related Art A prior art rolling mill of the generic type is known, for example, from European Patent 0 049 798. The rolls of one pair of this known rolling mill are provided with curved contours extending over the entire crowned length of both rolls and have a shape such that the two crowned contours complement each other exclusively in a specific relative axial position of these rolls. In a preferred embodiment in the prior art according to this reference, the two rolls of a pair are ground to be identically S-shaped and, in their installed position, are arranged to be turned through 180.degree. with respect to each other.
In the rolling of hot strip, this prior art rolling mill is intended to counteract thickness errors which occur over the width of the material to be rolled and which make the production of flat strip with a predefined strip profile more difficult. Thickness errors of this type are produced on the one hand because the rolls bend over their length under the rolling load and, on the other hand, by expansion of the rolls when they are in contact with the hot strip, which manifests itself through an increasing crown (thermal crown). Both the bending of the rolls over their length and the thermal crown alter the roll gap profile which causes--in accordance with the law of constant volume throughput per unit time--different strip lengthening to occur over the width of the material to be rolled which evidences itself as a waviness or saber formation in the strip.
A known solution for addressing this problem is to use contoured rolls. The contoured rolls are mutually axially displaced, given an appropriate ground roll crown, which enables the setting of different roll gap cross sections and also to compensate for the changes in the roll crown. Thus, for example, an S-shaped ground contour of the rolls, and the installed position of the opposite roll, rotated through 180.degree., in the prior art, make it possible, by mutual axial displacement of the rolls, to set both an equidistant and a convex or concave roll gap profile.
With the known solution, a clear advantage was achieved in that a radius of curvature was selected such that bending of the rolls under the rolling force was compensated for by the convex camber so that a flat strip was produced in the ideal case. However, the disadvantage of this solution is that the shape of the convex camber applies only to a specific bend caused by a specific rolling force and width of material to be rolled. Thus, if the rolling force or the width of the material to be rolled changed, different ground-crown rolls had to be used with a specific convex shape drawn to the particular application. In addition, this known solution does not compensate for the thermal expansion of the rolls.
The prior art solution using S-shaped contoured rolls permits a certain compensation of the changes in the roll crown which arise under the influences of load and temperature. However, it has the disadvantage that the curved roll crowns change as a result of thermal influences and wear and are difficult to produce. The curvature are asymmetrical in relation to the roll center, given essentially symmetrical bending of the rolls and a symmetrical strip, and in addition to the thermal expansion, the asymmetry of the curve causes asymmetrical wear which cannot be compensated for by displacing the rolls.